Immune activation secondary to microbial co-infections or vaccinations is known to induce high levels of viremia in HIV-infected individuals. In order to investigate the effects of immune activation on the pathogenesis of HIV disease, we set up an in vitro system that mimics in vivo antigen presentation; in this system, CD4+ T cells are co-stimulated with both CD3 crosslinking and CD28/B7 interactions. Using this system, we found that signaling through the two co-stimulatory molecules appears to have distinct effects on HIV infection. Co-stimulation with CD3 and B7-1 rendered the cells susceptible to, while co-stimulation with CD3 and B7-2 rendered the cells relatively resistant to HIV. Furthermore, through supernatant swapping experiments, we found that this effect is due, at least in part, to soluble factors secreted by B7-2-stimulated cells. Next, we investigated interactions between several microbial pathogens or their products with HIV or its target cells using in vitro systems. We found that macrophages exposed to bacterial cell wall components became highly susceptible to T-tropic strains of HIV-1, which otherwise poorly replicate in macrophages, while these bacterial products support infection with M-tropic strains of HIV less favorably. Such a dichotomy is due to differential effects of bacterial products on expression of HIV co-receptors and differential effects of CC chemokines produced by macrophages stimulated with those bacterial products. Furthermore, we found that supernatants from macrophages exposed to bacterial cell wall components could reactivate replication of HIV from latently infected resting CD4+ T cells. Finally, cathepsin G, a neutrophil-derived serine protease, was found to enhance susceptibility of macrophages to HIV infection; this result indicates that not only microbial co-infection, but also the inflammatory response to such an infection, may create an environment that is favorable for HIV replication.We also investigated whether human T-lymphotropic virus type I (HTLV-I) infection mediates any effect on the susceptibility of neighboring uninfected cells to HIV-1, since HTLV-I co-infection may accelerate progression of HIV disease. We have demonstrated that supernatants from HTLV-I-infected cells have dichotomous effects on HIV infection, depending on the virus strain tested: upregulation of T-tropic HIV-1 replication and suppression of M-tropic HIV-1 replication was observed. The HTLV-I supernatants contained several factors that variably influence HIV-1 infection, such as HTLV-I transactivator Tax protein, CC chemokines, and HTLV-I virions. Tax increased HIV envelope-mediated fusogenicity of the cells, and upregulated promoter activity of HIV-1 co-receptors CCR5, CXCR4, and STRL33, as well as the CC chemokine RANTES. These studies suggest that co-infection with a wide array of microbial pathogens (for example, bacteria and HTLV-I) generally leads to a more favorable environment for T-tropic HIV-1 replication and a less favorable or sometimes unfavorable environment for M-tropic HIV-1 replication, thereby accelerating the transition from an M- to T-tropic phenotype, which is indicative of disease progression in HIV infected individuals.